Transceivers such as these are used, for example, in the filtered mobile radio. In this case, multimode transceivers in particular are also used, which have a high integration density and require and extraordinarily flexible transceiver concept in order to satisfy the requirements for low power consumption, a small area requirement and the different mobile radios. In this case, each mobile radio communication system predetermines its own clock frequencies for the integrated analog/digital converters and for the digital signal processing unit. Thus, for example, a mobile radio which is operated on the basis of the GSM Standard (Global System for Mobile Communications) predetermines a clock frequency of 13 MHz or a multiple of this. A mobile radio which operates on the basis of the UMTS Standard (Universal Mobile Telecommunications/Telephony System) in contrast requires a clock frequency of 3.84 MHz. A mobile radio system which operates on the basis of the CDMA 2000 Standard (Code Division Multiple Access) requires a clock frequency of 3.6864 MHz while, in contrast, the GPS (Global Positioning System) requires a clock frequency of 10.033 MHz. Finally, the DAB (Digital Audio Broadcast/Broadcasting) system requires a clock frequency of 8.192 MHz. These examples clearly illustrate that there is no integer relationship between these different clock frequencies. However, a multimode or multisystem transceiver must be designed such that it can be used for all communications standards and, in particular, for the standards mentioned above. This means that the individual functional blocks in the transceiver must be suitable for use in different mobile radio communications systems.
A further problem in the development of an integrated multimode transceiver circuit is that, in addition, the clock frequency must at the same time be sufficiently low but the multiples of the clock frequency, that is to say their harmonic frequencies, do not fall in the desired signal band of the receiver or in the intermediate frequency band. The harmonic frequencies of the clock frequency must also not appear in the signal spectrum of the transmission signal. Furthermore, the integrated transceiver circuit must be designed such that reception interference in the receiver path caused by the harmonic clock frequencies of the parasitic modulation of the voltage controlled oscillator (VCO) is outside the specified frequency bandwidth of the mobile radio system. On the other hand, however, the clock frequency should be high enough in order to allow increased oversampling for the analog/digital converters, which may be in the form of switched capacitor (SC) sigma delta converters or continuous time (CT) sigma delta converters.
Because some local oscillators and the clocks for the analog/digital and digital/analog converters are integrated on the transceiver chip, the frequencies and clock frequencies must be planned particularly carefully. This is necessary in particular in order to avoid undesirable interference in the desired RF or IF band, or in the base frequency band. Furthermore, the required signals must be derived from the main clock in a manner which allows the current that is drawn to be kept as small as possible, the chip area to be minimized, and a flexible solution to be offered.
Until now, the clock for the analog/digital converters has been obtained from the clock signal from a crystal oscillator or from the harmonic frequencies of the clock signal from the crystal oscillator, with the crystal oscillator being arranged outside the integrated circuit. As an alternative to this the problem has until now been solved by providing a separate frequency generator, comprising a voltage controlled oscillator and a phase locked loop, specifically for this purpose outside the integrated circuit.
A method and an appliance for production and processing of quadrature signals are specified in the document DE 696 16 136 T2. This document describes a transceiver with a receiver in which two oscillators are provided in order to produce local frequencies. A/D converters, which operate at a sampling frequency fs, are connected via low-pass filters.
One object of the invention is to specify an integrated transceiver circuit which can be used for different clock frequencies, without interference from the harmonic frequencies produced by the clock frequencies.